It is known to provide a tape drive having data compression hardware (a DC drive) so that, as data arrives from a host, it is compressed before being written to tape thus increasing the tape storage capacity. A DC drive is also able to read compressed data from a tape and to decompress the data before sending it to a host.
There is more than one type of data compression. For example, removing separating marks such as record marks and file marks from user data and storing information regarding the separation marks in an index compresses the data. However, references to data compression (DC) in this specification will mean removing redundancy from the data (eg. replacing the user data by codewords form which the original data can be recovered). There are various algorithms which are used for data compression and it is envisaged that each of these algorithms will be given a universally recognized number. In practice, the application of a data compression algorithm can sometimes lead to expansion of the data (eg. if the data has previously been compressed).
In a DC drive, data compression and decompression may be carried out by a single DC chip. It is possible for a DC drive to contain more than one type of DC chip so that the drive may be able to perform data compression according to more than one data compression algorithm. An alternative approach is to perform data compression and data decompression in a host using the appropriate software.